LTC1197 LTC1199 - Linear Technology

7/98/184

Micropower MSOP 10-Bit ADC Samples at 500ksps

Design Note 184

Guy Hoover, Marco Pan and Kevin R. Hoskins

INTRODUCTION

The LTC

1197/LTC1199 10-bit serial ADCs offer small

size, low power operation and fast sample rates with good

AC and DC performance. These parts are ideal for low

power, high speed and/or compact designs. This Design

Note discusses the features and performance that make

the LTC1197/LTC1199 excellent choices for such new

designs.

FEATURES

Smallest Size (MSOP)

The LTC1197/LTC1199 are among the smallest ADCs

available. The LTC1197/LTC1199’s serial interface allows

the use of 8-pin MSOP and SO packages. Although the SO

package consumes little area, the MSOP package reduces

the small footprint even further. These are some of the ﬁrst

ADCs available in the MSOP package, which is about half

the size of the SO-8.

3V or 5V Supplies

The LTC1197/LTC1199 are 5V parts (V

= 4V to 9V for the

LTC1197 and 4V to 6V for the LTC1199). Also available for

use in 3V systems are the LTC1197L and the LTC1199L

= 2.7V to 4V). Designed for use in mixed-supply

systems, these devices operate flawlessly even when the

the digital input is greater than the V

voltage. This is

useful in systems where the ADC is running at a lower

supply voltage than the processor. If the ADC is running at

a higher supply voltage than the processor, the ADC serial

data output voltage can easily be decreased to a level

appropriate for the processor.

PERFORMANCE

Micropower Performance with Auto Shutdown

at Full Speed

Running continuously, the LTC1197L consumes only

2.2mW at the maximum sampling rate (25mW for the

LTC1197). The power consumption drops dramatically, as

High Speed Capability

Even though the LTC1197/LTC1199 are capable of

micropower operation, they are able to sample at rates of

up to 500kHz. These parts can also digitize fast input

signals up to the Nyquist frequency (250kHz for the

LTC1197) with over nine effective number of bits (ENOBs).

Good DC and AC Specs

The DC speciﬁcations of these parts are very good.

Linearity (both INL and DNL) is typically 0.3LSB with a

maximum spec of 1LSB. Offset is speciﬁed at 2LSBs

shown in Figure 1, at lower sampling rates. The formula for

calculating power consumption is:

• I

• t

CONV

•

where P

is the power consumption, V

is the supply

voltage, I

is the supply current while the conversion is

occurring, t

CONV

is the conversion time and f

is the sample

rate. As you can see from the formula, lowering f

reduces

the power consumption linearly. It is also important to

minimize t

CONV

by clocking the ADC at its maximum rate

during the conversion. In this way, the total power is less

because the device is on for a shorter period of time.

, LTC and LT are registered trademarks of Linear Technology Corporation.

SAMPLING FREQUENCY (kHz)

0.01

SUPPLY CURRENT (µA)

100

1000

10000

100

DN184 F01

0.1 0.1 110 1000

= 5V, 

CLK

= 7.2MHz

= 2.7V, 

CLK

= 3.5MHz

Figure 1. The LTC1197/LTC1199 Reduce Their Supply

Current Consumption and Save Power When Operating

at Lower Sample Rates

 LINEAR TECHNOLOGY CORPORATION 1998

dn184f LT/TP 0798 370K • PRINTED IN THE USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

●

FAX: (408) 434-0507

●

www.linear-tech.com

For literature on our A-to-D Converters,

call 1-800-4-LINEAR. For applications help,

call (408) 432-1900, Ext. 2453

(max) and gain error is speciﬁed at 4LSBs (max). These

speciﬁcations are guaranteed over the full temperature

range of the part. Both commercial and industrial tem-

perature range versions are available.

AC performance is equally impressive. S/(N + D) is typically

60dB (58dB for the L version). THD is typically

–64dB (–60dB for the L version) and the peak harmonic or

spurious noise is typically –68dB (–63dB for the L ver-

sion). An FFT of the LTC1197’s conversion performance is

shown in Figure 2.

tion of low level transducer outputs, which can save board

space and the cost of a gain stage.

With its software-selectable 2-channel MUX, the LTC1199

is capable of measuring either one differential or two

single-ended input signals. Both parts have a built-in

sample-and-hold.

Serial I/O

The LTC1197/LTC1199 are hardware and software com-

patible with SPI and MICROWIRE

protocols using either

3- or 4-wire serial interfaces. This compatibility is achieved

with no additional circuitry, allowing easy interface to many

popular processors.

Battery Current Monitor

FIgure 3 shows a 2.7V to 4V battery current monitor that

draws only 45µA from the battery it monitors. Supply

current is conserved by sampling at 1Hz and using the

LTC1152’s shutdown pin to keep the op amp off between

conversions. The LTC1197 automatically shuts down after

a conversion. The circuit can be located near the battery,

serially transmitting data to the microprocessor.

CONCLUSION

Conserving space and power, the LTC1197/LTC1199 have

a small footprint and are capable of micropower operation.

They have a versatile, SPI/MICROWIRE compatible serial

interface. The adjustable reference input, 2-channel, soft-

ware-selectable MUX and 5V or 3V operation increase this

ADC family’s versatility. When this versatility is combined

with the high conversion rate and good DC and AC perfor-

mance, you can see why these ADCs are good choices for

low power, high speed and/or compact designs.

FREQUENCY (kHz)

AMPLITUDE (dB)

–40

–20

–30

–10

DN184 F02

–60

–80

–50

–70

–90

–100 50 100 150 200 250

SMPL

= 500kHz

= 97.045898kHz



Figure 2. The LTC1197’s Typical 60dB SINAD Shown

in the FFT Curve is Among the Best and Translates

into 9.7 Effective Bits

MICROWIRE is a trademark of National Semiconductor Corporation.

–

LTC1152

SHDN

240k 56k TO

µP

100Ω

0.005Ω

2A FULL

SCALE

500pF

2.7V

TO 4V

1µF

0.1µFLT1004-1.2

DN184 F03

1

2

3

8

7

6

CS

+IN

–IN

GND



CLK

OUT



REF

LTC1197L

L

O

A

D



Figure 3. This 0A to 2A Battery Current Monitor Draws Only 45µA from a 3V Battery

Flexible Inputs

The LTC1197 has a single differential input and a wide

range reference input. The reference input allows the full

scale to be reduced to as low as 200mV. This translates to

an LSB size of only 200µV. Combined with the high

impedance of the analog input, this allows direct digitiza-